This invention relates to an improved protective helmet and a method and apparatus for making such a helmet. More particularly, the present invention relates to a helmet, and method and apparatus for making the same, having an undercut portion and perpendicularly directed holes which may be formed with a molding process.
Although protective helmets have long been essential equipment in contact sports such as baseball and football, they are finding more favor in non-contact sports such as bicycling, skateboarding and skating. In many of these principally non-contact sports, a potential impact to the wearer of the helmet occurs when the wearer falls down, striking his head on the ground or other object. Thus, helmets used in such non-contact sports are ideally designed to protect as much of the wearer's head as is possible.
A popular design for such a helmet is a compound curved surface having a generally elliptical hemispherical shell shape. Because of manufacturing constraints discussed below, most such helmets do not exceed 180 degrees of curvature in any single direction.
Even though such hemispherically shaped helmets cover a major portion of the skull, in use they can lack effectiveness. A helmet having the general shape of a hemispherical shell, with only 180 degrees of curvature, will sit on the top of the wearer's head. A retention system, usually in the form of a chin strap, must be employed to keep the helmet attached to the head in the event of a fall. Without a retention system, the helmet is only held on the wearer's head by the force of gravity.
However, the motion of the wearer's body during a fall which results in any substantial impact to the head is typically quite turbulent. With any significant turbulence of the body, the retention system may be incapable of preventing the helmet from shifting either down over the forehead, thereby leaving the back of the head exposed, or shifting rearwardly, leaving the forehead exposed. In either situation, a portion of the head which would otherwise be covered by the helmet is left exposed.
Even with an adequate retention system, properly adjusted by the wearer of the helmet, a helmet whose compound curved surface does not extend beyond 180 degrees still leaves portions of the head exposed. These include the portion of the skull inferior to the occipital lobe, including particularly the mastoid bone --the area of the cranial cavity where a basil skull fracture is most likely to occur.
One way of providing for additional coverage by the helmet is to extend the curvature of the helmet beyond 180 degrees, providing the helmet with a "backdraft" or "undercut," and allowing the helmet to cover the area inferior to the occipital lobe. An additional advantage of such a design is that the helmet, rather than just sitting on the head, is secured to the head by the undercut. When a helmet with an undercut is employed with a retention system, shifting of the helmet as a result of agitation of the head during a fall is minimized.
Protective helmets for non-contact sports such as bicycling, skateboarding and skating are typically made of expanded, non-resilient polymers such as expanded polystyrene (EPS). EPS consists of plastic cells that have been bonded together in the shape of a helmet during the molding process. EPS is currently the material of choice for such helmets because it is lightweight and has excellent shock attenuating properties
When subjected to an impact, the cells of an EPS helmet will "crush," or permanently deform, thereby protecting the wearer of the helmet by attenuating the energy of the impact. However, the inelasticity of EPS that allows for such good shock attenuation properties has some disadvantages.
For example, helmets made of EPS are rigid, inelastic, and have very little flexibility. Because EPS helmet liners are rigid and inflexible, they tend to break or crack on substantial impact. Therefore, helmets made of EPS are generally considered to be "singleimpact" helmets because they are no longer effective after a single substantial impact and must thereafter be replaced.
EPS helmets must typically be employed with a covering surrounding the EPS liner. In the event the EPS liner breaks into pieces upon impact, the cover preserves the integrity of the helmet. Coverings used on EPS helmets range from high-density plastic shell covers to fabric covers designed to merely hold the pieces together in the event of a fall. Regardless of the cover employed, utilizing a covering in connection with an EPS helmet adds cost and complexity to the manufacturing of the helmet.
Another significant disadvantage arising out of the inelasticity of EPS is that its use imposes limits on the shapes of helmets which can be molded. EPS helmets are generally made by a foam molding injection process. A typical mold for an EPS helmet liner has a core and a cavity. The space between the core and the cavity defines the shape of the helmet. The core is generally hemispherical in shape, and configured to roughly match the shape of the top of a human head. The cavity has generally the same shape as the core, but is slightly larger by a predetermined amount, thereby determining the wall thickness of the helmet.
When molding an EPS helmet, raw polystyrene beads containing a blowing agent are exposed to heat and pre-expanded to roughly the density desired in the helmet. The beads are then fed into the mold where they are further heated, causing additional expansion of the beads, forcing them to conform to the shape of the mold and causing the beads to bond to each other.
The mold is then allowed to cool, permitting the EPS to stabilize, at which time the core and the cavity are separated, usually by retracting the cavity from the core along a straight line of retraction, leaving the EPS helmet attached to the core. The helmet is then ejected from the core by a blast of air channeled into the core or with an ejector pin pushing the helmet off the core.
Care must be taken when ejecting the EPS helmet from the core to ensure that the helmet is not broken during the ejection process. To this end, the core of the mold is typically coated with a release agent, such as Teflon, to facilitate removal of the helmet.
Thus, the fragile nature of EPS helmets places certain limitations on the shapes of helmets which may be molded using EPS technology. For example, molding an EPS helmet with any significant undercut is impossible on a conventional mold. Because a helmet with an undercut has an inside curvature extending beyond 180 degrees, the helmet could not be removed from the core of the mold without breaking the helmet. In other words, an EPS helmet with an undercut would not have sufficient elasticity to permit the helmet to elastically deform a sufficient amount to be removed from the core of the mold.
Complex molds with collapsible cores have been utilized to manufacture EPS helmets with an undercut. Such molds are expensive and can be complicated to operate. Because steam is frequently utilized as a heat source in the molding process, such collapsible cores oxidize quickly and must be replaced with more frequency than non-collapsible molds.
Another attempt of the prior art to form an EPS helmet with an undercut is to mold the helmet in two sections and then attach the pieces. A significant disadvantage of this process is the additional cost of manufacture. Additionally, the impact-resistant properties of such a two-piece helmet are not as favorable as with a one-piece helmet.
The prior art has made attempts to design an EPS helmet providing similar protection to a helmet with an undercut. Such attempts have resulted in helmets having a generally hemispherical shell portion with a rear section extending downwardly in a straight line from the edge of the shell. Because such a design does not curve inwardly toward the head, the helmet is still susceptible to substantial movement during a fall.
An additional problem encountered in the manufacture of EPS helmets is configuring the helmet with holes in it to accommodate a helmet retention system or air vents. Virtually all sport helmets made today must be manufactured with a configuration of holes in them to accommodate both a helmet retention system and some form of an air ventilation system.
One method employed by the prior art for forming holes in an EPS helmet is to cut the holes in after the molding process with a hot knife or wire. The principal disadvantage to this procedure is that it can be extremely messy because melted EPS tends to accumulate on the knife and around the work station where the cutting is performed. Additionally, manually forming the holes in the helmet represents an additional step in the manufacturing process, resulting in increased manufacturing costs.
Attempts have been made to mold the holes in an EPS helmet at the same time the helmet is being molded. One method employed by the prior art is to configure the core and/or the cavity with projections to form the holes in the helmet as the part is molded. However, because of the extreme brittle nature of EPS, any projections extending into the part must necessarily be positioned along the line of retraction of the cavity. Otherwise the part cannot be removed from the mold without breaking. In a conventional molding process, this would result in the holes in the helmet being configured vertically.
Preferably, however, holes used for ventilation purposes are configured horizontally (as the helmet is worn) or are configured substantially perpendicular to a line tangent to the surface of the helmet at the hole. Because these positions are not along the line of retraction, they cannot be molded into an EPS helmet using a conventional molding process
Perhaps a more significant disadvantage of molding vertically positioned holes into the helmet is that the amount of material taken from the helmet is greater than necessary. For example, when attempting to mold a hole in the front of the helmet with a mold projection which must extend vertically, a great deal of material must be displaced to form the hole. In some helmet designs, the additional material taken from the helmet would unacceptably weaken the helmet.
Another attempt made by the prior art to efficiently mold holes into EPS helmets is to employ a "sliding" core in which there are movable projections in the core which correspond in size to the holes to be formed in the helmet. When molding the helmet, the projections are inserted into the void between the core and the cavity before the polystyrene beads are introduced into the mold. After the part cools, the projections are retracted into the core before the core and the cavity are separated. Such sliding cores, however, are expensive to manufacture and, because they operate in a hot, moist environment, their useful life is only a fraction of that of a one-piece core.
From the foregoing, it can be seen that what is needed in the art is a protective helmet, and a method and apparatus for making the same, having good shock attenuating properties and which can be molded in one piece with an undercut by conventional molding processes.
It would also be an advancement in the art if a protective helmet could be provided, and a method and apparatus for making the same, in which holes positioned perpendicular to the surface of the helmet could be molded into the helmet without the use of sliding cores.
Such a helmet, and method and apparatus for making the helmet, is disclosed and claimed herein.